LeaseSet Key Persistence

Overview

This proposal is about persisting additional data in the LeaseSet that is
currently ephemeral.
Implemented in 0.9.18.

Motivation

In 0.9.17 persistence was added for the netDb slicing key, stored in
i2ptunnel.config. This helps prevent some attacks by keeping the same slice
after restart, and it also prevents possible correlation with a router restart.

There's two other things that are even easier to correlate with router restart:
the leaseset encryption and signing keys. These are not currently persisted.

Proposed Changes

Private keys are stored in i2ptunnel.config, as i2cp.leaseSetPrivateKey and i2cp.leaseSetSigningPrivateKey.